lora: shell: use strtoul instead of strtoll

Parsed value is expected to be in range (0, UINT32_MAX). Use strtoul
instead of strtoll, so we better match its range. In a tested
configuration this saves 380 bytes of flash with newlib and arm32
target.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>